1. Field of the Invention
The present invention relates to a superconductive magnet device and a magnetic resonance imaging apparatus (hereinafter referred to as MRI apparatus), and particularly, to a superconductive magnet device suitable for an open-type MRI apparatus that does not give a siege mentality to a patient body and the MRI apparatus using the same.
2. Description of the Related Art
Conventionally, an MRI apparatus that uses a superconductive magnet device and generates a static magnetic field is widely used especially in a field of a medical diagnosis, and an open-type MRI apparatus that does not give a siege mentality to a patient body is noticed as a biopsy apparatus because of its easy access to the patient body.
The MRI apparatus comprises a pair of cylindrical or elliptical upper and lower vacuum vessels, and has a magnetic space (imaging region) between the vacuum vessels. In each of the vacuum vessels is housed a coil vessel where a plurality of pairs of superconductive main coils different in diameter are housed together with a refrigerant. In the plurality of the pairs of the superconductive main coils are included at least one pair of superconductive mail coils and at least one pair of superconductive shielding coils so that a uniform static field is generated in a spherical or ellipse-spherical imaging region having a center within a gantry gap.
As the refrigerant housed in the coil vessels, liquid helium is used, and in order to reduce the consumption of the liquid helium, an ultra low temperature refrigerator for again condensing a vapor gas is used.
Generally, a refrigerator is installed at the upper center position of an upper vacuum vessel or above the position of a side support member for joining a pair of the upper and lower vacuum vessels from the need of condensing a vapor gas again (for example, see Japanese Patent Laid-Open Publication Nos. 2002-17705, 2005-237417, and 2005-185551).
Furthermore, as a service port having such a power lead for supplying an excitation current to superconductive main coils, a service port (Japanese Patent Laid-Open Publication No. H05-217743) that is attachable and detachable, and another service port (Japanese Patent Laid-Open Publication No. 2004-51475) having a supply port for supplying helium are disclosed.
By the way, in the conventional technology recited in the above JP 2004-51475, a refrigerator, a power lead, and a supply port are configured to be disposed adjacently, and if such the configuration is adopted in apparatuses shown in the JP 2002-17705, the JP 2005-237417, and the JP 2005-185551, it is not possible to simultaneously perform a maintenance work relating to the refrigerator, the attachment and detachment work of the power lead, and the supply work of liquid helium through the supply port. For example, the power lead is obstructive in the maintenance work relating to the refrigerator, and the refrigerator is obstructive in the attachment and detachment work of the power lead.
Furthermore, if a power lead port to which the power lead is connected and the refrigerator are disposed adjacently, there is a possibility that a combined whole shape thereof is jumboized and that the shape becomes complicated such that a port of the refrigerator cannot be configured to be cylindrical so as to match a contour of the refrigerator.
Furthermore, in such a power lead port as shown in the JP H05-217743 where the attachable and detachable power lead is used, because gas cooling is necessary while electricity is passed through the power lead port, dew condensation may occur in the power lead port due to the cooling effect of the cooling gas. In this case, if the refrigerator and the power lead port are disposed adjacently, there is a possibility that vacuum heat insulation of the superconductive magnet device is reduced because such defects are caused that a vacuum packing used for vacuum sealing of the refrigerator deteriorates, and that a moisture generated from dew condensation permeates the vacuum packing and a vacuum leak of the refrigerator occurs. If the vacuum heat insulation is reduced, there is also a possibility that it causes the decrease of the refrigerant and finally comes into coil quench of the superconductive magnet.
Consequently, there is a need for a superconductive magnet device and an MRI apparatus, wherein the maintenance work of a refrigerator and the attachment and detachment work of a power lead port may be performed simultaneously without a trouble.